The present application relates to a data transmission device of a reflected wave transmission system for transmitting data using a modulated reflected wave generated by modulating an unmodulated carrier supplied from a data reader. The present application relates more particularly to a data transmission device for handling high speed data transmission by performing a primary modulation of a subcarrier followed by a secondary modulation of a reflected wave.
More specifically, the present application relates to a data transmission device for providing faster reflected wave data transmission by using a primary modulation which can carry binary or multi-level amplitude information. The present application relates more particularly to a data transmission device for suppressing a modulated reflected wave from spreading into side lobes during a primary modulation of a subcarrier.
A non-contact communication system called RFID (Radio Frequency Identification) is known as a communication system adapted to wirelessly send data without having any own radio wave generating source. The RFID includes a tag and reader. The tag is a passive device, receiving radio wave from the reader as an energy source so that information is read from the tag.
Although referred to in various manners such as ID System and Data Carrier System, the RFID System or RFID for short, is a globally common name. The RFID can be translated to mean Identification System Using High Frequency (Radio Frequency) in Japanese.
Non-contact communication techniques used for the RFID system are capacitive coupling, electromagnetic induction and radio communication. Of these techniques, the RFID system based on radio communication includes a reflector and reflected wave reader. The reflector sends data on a reflected wave generated by modulating a received unmodulated carrier. The reflected wave reader reads data from the modulated reflected wave from the reflector. This system performs reflected wave transmission called “backscatter.”
Upon receipt of an unmodulated carrier from the reflected wave reader, the reflector modulates data on the reflected wave of the carrier, for example, by changing the antenna load impedance. That is, the reflector demands no carrier generating source. This makes it possible for the reflector to ensure low power consumption in data transmission. Upon receipt of such a modulated reflected wave, the reflected wave reader demodulates and decodes the received reflected wave to obtain transmitted data.
On the other hand, an antenna switch adapted to change the antenna load impedance of the reflector may be incorporated in a circuit module and configured with CMOS (Complementary Metal Oxide Semiconductor) transistors. However, the switch provides low power consumption and fast switching if it is separate from the circuit module and configured with gallium arsenide (GaAs) IC (Integrated Circuit). The latter offers improved data transmission rate using reflected wave modulation and keeps the power consumption to several tens of μW or less. Considering that wireless LAN consumes several hundreds of mW to several W during communication, reflected wave communication can be said to be by far superior in performance to typical wireless LAN in terms of average power consumption (refer, for example, to Japanese Patent Laid-Open No. Hei 10-209914, hereinafter referred to as Patent Document 2).
A reflector-equipped terminal does nothing but reflect the received radio wave. As a result, the terminal is not regarded as a radio station. Instead, it is treated as a device not subject to laws or regulations relating to radio wave communication. Further, other types of non-contact communication systems such as those based on electromagnetic induction employ frequencies between several MHz to several hundreds of MHz (e.g., 13.65 MHz). In contrast, systems based on reflected wave communication can provide high speed data transmission using, for example, the high frequency 2.4 GHz (microwave) band, which is referred to as the ISM (Industry, Science, and Medical Band).
For example, a reflector is built into a terminal device whose power consumption should be kept to a minimum such as digital camera, video camcorder, mobile phone, mobile information terminal, or portable music player. A reflected wave reader is built into host equipment which includes a stationary household electric appliance such as television set, monitor, printer, PC, VCR, or DVD player. This makes it possible to upload image data captured with a camera-equipped mobile phone or digital camera to the PC via a reflected wave transmission line so that such image data can be accumulated, displayed or printed.
In reflected wave transmission, the frequency of the carrier from the reader is normally the same as the center frequency of the reflected wave. This forces the reader to handle sending and reception at the same frequency. As a result, the receiving section is prone to DC offset and sender noise due to coupling loop interference caused by a sending signal. This makes it difficult to expand the transmission distance. Therefore, the isolation between the sender and receiver is a problem to be addressed. On the other hand, the reflected wave transmission employs ASK (Amplitude Shift Keying) or PSK (Phase Shift Keying) for modulation in almost all cases, making it difficult to provide faster transmission. A solution proposed to solve these problems is to modulate data on a subcarrier, for example, by PSK, QPSK or 8PSK as a primary modulation, followed by modulation of the reflected wave by changing the antenna load impedance using an antenna switch (refer, for example, to Patent Document 2).
In the above reflected wave transmission adapted to perform a primary modulation of a subcarrier, however, the antenna switch is turned on and off using a binary digital signal carried by the subcarrier. As a result, only relatively slow subcarrier modulation schemes, such as ASK, PSK, QPSK (Quadrature PSK) and 8PSK, which demand only binary amplitude information, can be used. In other words, modulation schemes carrying binary or multi-level amplitude information such as 16QAM (Quadrature Amplitude Modulation), 64QAM and OFDM (Orthogonal Frequency Division Multiplexing) cannot be used.
Further, even when a modulation scheme such as ASK, PSK, QPSK or 8PSK is used, it is impossible to modulate the reflected wave using the antenna switch if the digital modulated signal is limited in bandwidth and converted into analog form. In this case, the frequency spectrum of the reflected wave spreads out infinitely, raising concern that other systems in the neighborhood may be affected by interference. These problems will be described below.
FIG. 6 illustrates a configuration example of a reflected wave transmission system using a subcarrier. In FIG. 6, reference numeral 10 denotes a data transmission device adapted to send data by modulating the reflected wave of a received radio wave. Reference numeral 11 denotes a data reader adapted to receive the modulated reflected wave signal from the transmission device 10 to read data therefrom.
The data transmission device 10 is incorporated in a mobile device which serves primarily as a data transmission source such as digital camera or mobile phone. The same device 10 transmits moving image data and music data stored therein to the reader 11. As illustrated in FIG. 6, the transmission device 10 includes an antenna 100, a band-pass filter (BPF) 101, an antenna load selector switch 102, a subcarrier oscillator 103 having a frequency Fs and a subcarrier modulator 104.
One end of the antenna load selector switch 102 is grounded. The same switch 102 serves as a load of the antenna 102 and is short-circuited when turned on and open-circuited when turned off, thus allowing an unmodulated wave from external equipment to be PSK-modulated. The subcarrier modulator 104 controls the on/off state of the antenna load selector switch 102 using a digital modulated signal generated from sending data (TX_DATA). The subcarrier oscillator 103 generates a center frequency Fs used to modulate the subcarrier. Thus, the subcarrier modulator 104 can generate two modulated waves of an unmodulated carrier having a center frequency Fo received by the antenna 100. The two modulated waves respectively have center frequencies Fo+Fs and Fo−Fs which are upwardly and downwardly apart by the subcarrier frequency Fs from the unmodulated carrier at the center frequency Fo.
On the other hand, the band-pass filter 101 in the transmission device 10 is used to extract only one of the two modulated waves (Fo+Fs in the example shown). Thus, the same filter 101 passes the received radio wave at the center frequency Fo and the modulated reflected wave at the center frequency Fo+Fs. Here, the band-pass filter 101 need not be used. In this case, however, the frequency spectrum of the modulated reflected wave spreads out infinitely. Even when the band-pass filter 101 is used, it is unrealistic to hope for a frequency characteristic steep enough to attenuate the side lobes. As a result, the spread of the spectrum must be tolerated to a certain extent. Here, the center frequency Fo of the unmodulated carrier is assumed to be in 2.4 GHz band, and the subcarrier frequency Fs to be several tens of MHz.
On the other hand, the reader 11 includes an antenna 105, a sending section 108 adapted to send the unmodulated carrier Fo, and a receiving section 107 adapted to receive the modulated reflected wave Fo+Fs of the unmodulated carrier Fo from the transmission device 10. The reader 11 further includes a circulator 106 adapted to separate the sending and receiving sections 108 and 107 so as to simultaneously handle sending and reception with a single antenna 105. The reader 11 still further includes a baseband processing section 109 adapted to handle demodulation and communication control. The unmodulated carrier at the center frequency Fo is transmitted from the sending section 108 via the circulator 106 and antenna 105. This unmodulated carrier returns to the reader 11 in the form of a modulated wave at the center frequency Fo+Fs after being reflected by the transmission device 10. The carrier is received by the receiving section 107 via the antenna 105 and circulator 106. The carrier is then demodulated by the baseband processing section 109 for use as received data (RX_DATA).
Although mention was made of a unidirectional transmission from the transmission device 10 to the reader 11, it is practically common that control and user data is transmitted from the reader 11 to the transmission device 10. However, the latter data transmission is not directly related to the spirit of the present invention. Therefore, a detailed description thereof will be omitted in the present specification.
FIG. 7 illustrates the frequency spectrum of the reflected wave transmission system shown in FIG. 6. In FIG. 7, reference numeral 200 denotes an unmodulated carrier supplied from the reader 11. Reference numerals 201 and 202 denote modulated reflected waves of the unmodulated carrier generated by the data transmission device 10.
The system shown in FIG. 6 uses only one of the two modulated waves, namely, the wave with the center frequency Fo+Fs. Therefore, the band-pass filter 101 of the data transmission device 10 offers an attenuation characteristic as denoted by reference numeral 203 to attenuate the reflected wave signal. Thus, the same filter 101 passes the unmodulated carrier 200 with the center frequency Fo and the reflected wave 201 with the center frequency Fo+Fs. However, it is difficult to implement the band-pass filter capable of steeply attenuating the side lobes other than the main lobe of the reflected wave 201. As a result, the side lobes spread out to a certain extent as illustrated in FIG. 7.
[Patent Document 1]
Japanese Patent Laid-Open No. 2005-64822